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Atmosphere2014, 5, 342-369; doi:10.3390/atmos5020342
atmosphereISSN 2073-4433
www.mdpi.com/journal/atmosphere
Article
Mercury Plumes in the Global Upper Troposphere Observed
during Flights with the CARIBIC Observatory from May 2005
until June 2013
Franz Slemr1,*, Andreas Weigelt
2, Ralf Ebinghaus
2, Carl Brenninkmeijer
1, Angela Baker
1,
Tanja Schuck1,
, Armin Rauthe-Schch1, Hella Riede
1, Emma Leedham
1, Markus Hermann
3,
Peter van Velthoven4, David Oram
5, Debbie OSullivan
5, , Christoph Dyroff
6, Andreas Zahn
6
and Helmut Ziereis7
1 Atmospheric Chemistry Division, Max-Planck-Institut fr Chemie
(MPI), Hahn-Meitner-Weg 1,
D-55128 Mainz, Germany; E-Mails: [email protected]
(C.B.);
[email protected] (A.B.); [email protected] (T.S.);
[email protected] (A.R.-S.); [email protected]
(H.R.);
[email protected] (E.L.)2 Institute of Coastal Research,
Helmholtz-Zentrum Geesthacht (HZG), Max-Planck-Strae 1,
D-21502 Geesthacht, Germany; E-Mails: [email protected]
(A.W.);
[email protected] (R.E.)3 Leibniz-Institut fr
Troposphrenforschung (TROPOS), Permoserstrasse 15,
D-04318 Leipzig, Germany; E-Mail: [email protected] Royal
Netherlands Meteorological Institute (KNMI), P.O. Box 201,
NL-3730 AE De Bilt, The Netherlands; E-Mail: [email protected]
National Centre for Atmospheric Science, University of East Anglia
(UEA),
Norwich NR4 7TJ, UK; E-Mails: [email protected] (D.O.);
[email protected] (D.O.S.)6
Institute of Meteorology and Climate Research, Karlsruhe
Institute of Technology (KIT),Hermann-von-Helmholtz-Platz 1,
D-76344 Eggenstein-Leopoldshafen, Germany;
E-Mails: [email protected](C.D.); [email protected]
(A.Z.)7 Institut fr Physik der Atmosphre, Deutsches Zentrum fr
Luft- und Raumfahrt (DLR),
D-82230 Wessling, Germany; E-Mail: [email protected]
Current Affiliation: NRW State Agency for Nature, Environment
and Consumer Protection,
Recklinghausen, Germany Current Affiliation: Meteorological
Office, Exeter, EX1 3PB, UK
* Author to whom correspondence should be addressed; E-Mail:
[email protected];
Tel.: +49-8821-52595.
OPEN ACCESS
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Received: 24 February 2014; in revised form: 28 April 2014 /
Accepted: 30 April 2014 /
Published: 28 May 2014
Abstract:Tropospheric sections of flights with the CARIBIC
(Civil Aircraft for Regular
Investigation of the Atmosphere Based on an Instrumented
Container) observatory from
May 2005 until June 2013, are investigated for the occurrence of
plumes with elevated Hg
concentrations. Additional information on CO, CO2, CH4, NOy, O3,
hydrocarbons,
halocarbons, acetone and acetonitrile enable us to attribute the
plumes to biomass burning,
urban/industrial sources or a mixture of both. Altogether, 98
pollution plumes with
elevated Hg concentrations and CO mixing ratios were
encountered, and the Hg/CO
emission ratios for 49 of them could be calculated. Most of the
plumes were found over
East Asia, in the African equatorial region, over South America
and over Pakistan and
India. The plumes encountered over equatorial Africa and over
South America originate
predominantly from biomass burning, as evidenced by the low
Hg/CO emission ratios and
elevated mixing ratios of acetonitrile, CH3Cl and particle
concentrations. The backward
trajectories point to the regions around the Rift Valley and the
Amazon Basin, with its
outskirts, as the source areas. The plumes encountered over East
Asia and over Pakistan
and India are predominantly of urban/industrial origin,
sometimes mixed with products of
biomass/biofuel burning. Backward trajectories point mostly to
source areas in China and
northern India. The Hg/CO2 and Hg/CH4 emission ratios for
several plumes are also
presented and discussed.
Keywords:mercury; emission; air; pollution
1. Introduction
Mercury (Hg) is emitted by natural and anthropogenic processes,
and because of its rather long
atmospheric lifetime of one year, it can be transported over
long distances [1,2]. After oxidation and
deposition, part of it can be transformed to highly neurotoxic
methyl mercury. The latter is thenbio-accumulated in the aquatic
food web and may harm both human populations and fauna, which
are
dependent on fish [3,4]. Emissions from different natural and
anthropogenic processes, such as
volcanic emissions, emission from the oceans, from soils, coal
and biomass burning, as well as many
other anthropogenic activities, have been estimated, and
spatially and temporally resolved emission
inventories have been calculated from the emission factors
obtained in these studies (e.g., [515]).
Despite all these efforts, the emission estimates are still
quite uncertain, especially those related to
natural sources and anthropogenic emissions in rapidly
developing countries in East and South-East
Asia [9,13,16,17]. Thus, more data on mercury emissions are
required, and the existing inventories
need to be evaluated by measurements.
Direct measurements of emissions by techniques, such as a mass
balance technique or using an
artificially emitted tracer substance [18], are complex and
expensive. The mass balance technique
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measures the fluxes in and out of a chosen source area and
calculates the emissions as a difference
between them. For a middle-sized city, it requires the use of
several aircraft equipped with precise
chemical and meteorological instrumentation to resolve small
differences of large fluxes.
Alternatively, an artificial tracer, such as SF6, is emitted in
an area under investigation and the
emission of the target substance is calculated from the known
emission of the artificial tracer and the
correlations of the target substance concentrations with those
of the tracer. Both techniques have been
successfully used to determine emissions of CO and NOy of a
middle-sized city [18], but they can
hardly be scaled up to larger areas. A determination of emission
ratios of two substances from their
concentrations in the plumes even of large areas is
experimentally much more amenable [1820].
Consequently, emission ratios are promising to be the most
practicable way to evaluate the consistency
of an emission inventory of one substance with an inventory of
another substance [21]. Emission ratios
can also be used to constrain the lesser known emissions of one
substance using the better known
emissions of another substance [21,22].The CARIBIC (Civil
Aircraft for Regular Investigation of the Atmosphere Based on
an
Instrumented Container) observatory is a long-term project aimed
at the monitoring of atmospheric
composition and its changes by using an instrumented freight
container flown on-board a passenger
aircraft during intercontinental flights [23]. It started in
1997 and, apart from an interruption between
2002 and 2004, has been operational continuously over more than
15 years. Despite cruising most of
the time at altitudes from 10 to 12 km, plumes of polluted air
lifted mostly by convection or warm
conveyor belts [24,25] are frequently encountered in the
tropospheric sections of the flights.
Here, we report on plumes with elevated mercury concentrations
observed during CARIBIC flights
since May 2005, when a mercury instrument was installed until
June 2013. Rich ancillary data onother gases, such as carbon
monoxide (CO), carbon dioxide (CO2), methane (CH4), total
reactive
nitrogen (NOy), hydrocarbons, halocarbons and on atmospheric
aerosol, enable a detailed
characterization of the plumes, its attribution to the emission
processes and, in combination with
backward trajectories, an approximate localization of the
emissions. Correlations of Hg with CO, CO2
and CH4 provide Hg/CO, Hg/CO2 and Hg/CH4 emission ratios, which
may help to constrain the
estimates of mercury emissions using the CO, CO2and CH4emission
inventories [21].
2. Experimental Section
Since December 2004, a new CARIBIC measurement container [23]
on-board an Airbus 340600 of
Lufthansa has been flown monthly on transcontinental flights.
The corresponding routes (until June 2013)
are shown in Figure 1, and the complete list of flights can be
found at www.caribic-atmospheric.com.
Typically, a sequence of 4 consecutive intercontinental flights
is executed every month. The modified
freight container (gross weight: 1.5 metric tons) holds 15
automated analysers for the in situ
measurements of mercury concentrations and mixing ratios of CO,
O3, NO, NOy, CO2, total (including
cloud droplets) and gaseous water, oxygenated organic compounds
and concentrations of fine particles
(three counters for particles with diameters >4 nm, >12 nm
and >18 nm, all up to 2 m), as well as one
optical particle counter for particles with diameters of 130900
nm. In addition, air and aerosolsamples are taken in flight and
subsequently analysed in the laboratory for greenhouse gases,
halocarbons, non-methane hydrocarbons (NMHCs) and particle
elemental composition and
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morphology, respectively [23]. In May 2010, several instruments
were upgraded and new instruments
were added. In the context of this paper, the most important
change was the addition of a whole air
sampler with a capacity of 88 samples and of an instrument for
continuous measurements of CH 4
(Fast Greenhouse Gas Analyzer, Los Gatos Research, [26]). With
the new whole air sampler, the
measurement frequency of greenhouse gases [27] and hydrocarbons
[28] could be increased from 28 to
116 measurements per flight sequence. Halocarbon measurements
(except for CH3Cl, which can be
determined by both hydrocarbon and halocarbon analytical
methods) were unaffected, due to the
limited volume of air available for analysis in the new
sampler.
Figure 1. The tracks of 328 CARIBIC (Civil Aircraft for Regular
Investigation of the
Atmosphere Based on an Instrumented Container) flights from May
2005 until June 2013.
The colours denote the classification of destination airports
used in this paper: green, East
Asia; yellow, South Asia; light blue, Africa; dark blue, South
America; red,
North America.
The air and aerosol inlet system and instrument tubing are
described in detail by
Brenninkmeijeret al.[23] and Slemret al.[29]. Briefly, the trace
gas probe consists of a 3-cm inner
diameter diffuser tube with a forward facing inlet orifice of 14
mm in diameter and an outlet orifice of
12 mm in diameter, providing an effective ram pressure of about
90170 hPa, depending on cruising
altitude and speed. This ram pressure forces about 100 L/min of
ambient air through a PFA tubing
heated to 40 C (a 3 m-long, 16-mm ID PFA-lined tube connecting
the inlet and the container and
1.5 m-long, 16-mm ID PFA tubing within the container to the
instrument manifold). The sample air for
the mercury analyser is taken at a flow rate of 0.5 L(STP,i.e.,
at standard temperature of 273.15 K and
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pressure of 1013.25 hPa)/min from the manifold using 4-mm ID PFA
tubing heated by the energy
dissipated in the container to ~30 C. The arrangement similar to
that described by Talbot et al.[30]
was optimized to transmit highly surface reactive HNO3[31] and
can thus be presumed to facilitate the
transfer of gaseous oxidized mercury (GOM), as well [30]. The
large flow through the trace gas
diffuser inlet tube of more than 2000 L/min and perpendicular
sampling at much smaller flow rates of
about 100 L/min discriminate against particles larger than about
one micrometre in diameter (50%
aspiration efficiency [32]). Consequently, all smaller particles
and, thus, the major fraction of particle
mass in the upper troposphere will be transported to the
manifold in the container.
The mercury instrument, which is based on an automated dual
channel, single-amalgamation, cold
vapour atomic fluorescence analyser (Tekran-Analyzer Model 2537
A, Tekran Inc., Toronto, ON,
Canada), is described by Slemret al.[29]. The instrument
features two gold cartridges. While one is
adsorbing mercury during a sampling period, the other is being
thermally desorbed using argon as a
carrier gas. Hg is detected using cold vapour atomic
fluorescence spectroscopy (CVAFS). Thefunctions of the cartridges
are then interchanged, allowing continuous sampling of the incoming
air
stream. A 45-mm diameter PTFE pre-filter (pore size 0.2 m)
protects the sampling cartridges against
contamination by particles that pass through the inlet system.
The 0.5 L(STP)/min of air sample, typically
at 200300 hPa, is compressed to about 500 hPa, needed to operate
the instrument with its internal
pump. Extensive laboratory tests of this PTFE diaphragm pump
(KNF-Neuberger, Model N89KTDC)
did not show either any contamination of the system with Hg or
Hg losses. To avoid the contamination
of the instruments and of the tubing connecting the sampling
manifold with the instruments during
ascents and descents in polluted areas near airports, the
sampling pumps are activated only at an
ambient pressure below 500 hPa. Consequently, no measurements
below an altitude of about 5 kmwere made.
Initially, the instrument was operated with a gas mixture of
0.25% CO2in argon, which also is used
for the operation of the CO instrument. Because the addition of
CO2to argon reduced the sensitivity of
the fluorescence detector by ~35%, the instrument was run
initially with a 15-min sampling time
(corresponding to a ~225 km-flying distance) until March 2006
(Flight 145) and with 10 min until
June 2007 (Flight 197). Since August 2007, the CO2has been
removed from the gas mixture using a
tube filled with an X10 molecular sieve. The corresponding
sensitivity gain enabled us to reduce the
sampling interval to 5 min (corresponding to a ~75-km flying
distance). The instrument is calibrated
after every other month in the laboratory by ~48 h of parallel
operation to a well-calibrated identical
instrument. A detection limit of ~0.1 ngHgm3 and a
reproducibility of about 0.05 ngHgm3 is
achieved at our operating conditions. To improve the detection
limit and reproducibility of the
measurements, we returned to 10-min sampling in August 2011
(Flight 349). For this paper, the data
from May 2005 to June 2013, were analysed. All mercury
concentrations are reported in ngHgm3(STP,
i.e., at 1013.25 hPa and 273.15 K).
Speciation experiments on-board the CARIBIC container, where
gaseous oxidized mercury (GOM)
was removed in the instrument using a KCl or soda lime trap
upstream of one of the gold cartridges,
showed qualitatively that GOM (essentially Hg2+) is transmitted
through the inlet system to the
instrument and will be measured. A demonstration of a
quantitative transmission would require
capabilities to prepare GOM test mixtures at high flow rates and
to replicate the flight conditions
(i.e., 50 C, 900 kmh1), which is beyond the constraints imposed
by a commercial airliner.
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Temmeet al.[33] found the GOM transmission to be quantitative at
conditions similar to those in the
upper troposphere, i.e., low temperatures and dry air, which
allows us to assume the same for our
system. A definitive verification of this assumption has to wait
for an in-flight intercomparison with a
research aircraft with proven speciation capabilities. Newer
data on the gas-particle partitioning of
atmospheric Hg2+[34,35] suggest that particle bound mercury
(PBM, also mostly Hg2+) sampled near
the tropopause at temperatures of ~50 C will evaporate when
warmed up to ~+40 C during the
transport in the sampling tubing to the instrument. PBM on
particles that make it into the trace gas
inlet will thus be most likely also measured. Consequently, the
CARIBIC measurements approximate
the total mercury concentration in the troposphere. We note that
even if GOM concentrations in the
upper troposphere represent more than 1% or less of total
gaseous mercury concentrations typically
found in the boundary layer [36,37], its non-quantitative
transmission by our inlet system would not
substantially influence the results presented in this paper.
The plumes with elevated Hg concentrations showed, apart from a
few exceptions mentioned inSection 3.1, also elevated CO and,
sometimes, CO2 and CH4 mixing ratios. For these plumes, the
Hg/CO, Hg/CO2and Hg/CH4emission ratios were calculated by
bivariate least-squares correlations of
Hg with CO, CO2 and CH4 [38], respectively, which take into
account the uncertainties in both
variables. For these correlations, the continuously measured CO,
CO2and CH4were averaged over the
Hg sampling interval. The uncertainties of Hg, CO, CO2 and CH4
measurements were set to
0.05 pgm3, 1 ppb, 0.05 ppm and 3 ppb, respectively. The
underlying assumptions in the emission
ratio calculations are: (1) that none of the correlated
substances are lost during the transport from the
source to the point of encounter by chemical reactions; (2) that
the emission ratios are nearly constant
during the observation interval; and (3) that the plume is
embedded in a homogeneous air mass, i.e.,that the Hg concentration
and CO, CO2or CH4mixing ratios before and after the plume are
nearly the
same [20]. Assumption (1) is fulfilled, as the transport times
(ranging from a few days to about
one week) are much shorter than the atmospheric lifetime of our
target compounds (CO has the
shortest lifetime of ~2 months, with a local lifetime in the
tropics being several weeks). Assumptions
(2) and (3) are probably fulfilled for a majority of smaller
plumes. The large plumes stretching over
thousands of kilometres north and south of the intertropical
convergence zone (ITCZ) during the
flights to South Africa are superimposed on a north-south Hg
gradient, which violates Assumption (3).
Sometimes, large overlapping plumes with different sources in
different areas are sampled, also violating
the Assumption (2). Consequently, even statistically significant
Hgvs.CO, CO2and CH4correlations may
sometimes provide biased Hg/CO, Hg/CO2or Hg/CH4emission ratios
in the case of the African flights.
Meteorological analyses for all CARIBIC flights are provided by
KNMI (Royal Netherlands
Meteorological Institute) at http://www.knmi.nl/samenw/CARIBIC.
Trajectories were calculated at 3-min
intervals along the flight track for each flight with the KNMI
trajectory model, TRAJKS [39], using
data from ECMWF (European Centre for Middle Weather Forecast)
data.
3. Results and Discussion
3.1. Overview
Compared to the search for plumes at a ground station [21], the
processing of the CARIBIC data is
complicated by the variability of the data over large distances
in the upper troposphere and by the
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frequent changes of tropospheric and stratospheric air masses.
Figure 2 shows an overview of the data
from Flight 158 from Frankfurt to Guangzhou on 31 July and 1
August 2006. The aircraft flew in the
troposphere until ~23:00 UTC and then in the stratosphere until
about 1:45 UTC on 1 August.
The stratospheric section is evidenced by the high potential
vorticity and O3mixing ratio, as well as
the low CO mixing ratio shown in the upper two panels of the
data time series. In the tropospheric
section after about 1:45 UTC on 1 August 2006, mercury
background concentrations vary between
about 1.25 and 1.35 ngm3(the second panel from the top of the
data time series). Three events with
elevated Hg concentrations, denoted as A, B and C, with peak Hg
concentrations of 1.55, 1.5 and
2.3 ngm3are observed at about 2:40, 4:50 and 6:00 UTC,
respectively, on 1 August 2006. All events
are accompanied by elevated CO (the second panel from the top),
NOy, H2O (middle panel) and
acetone (bottom panel). We base our approach on the visual
inspection of the data overview plots of
each flight for the coincident occurrence of elevated Hg
concentrations with elevated CO mixing
ratios. Plumes identified in this way are cross-checked using
variations of other tracers foranthropogenic emissions, such as NOy
(middle panel), acetone (bottom panel), CH4(the second panel
from the bottom), non-methane hydrocarbons (not available for
this flight) and halocarbons (the
second panel from the bottom). Humidity and cloud water content
(determined as the difference
between total water content and the water vapour mixing ratio)
as tracers for convective processes are
also sometimes useful.
An event within the stratospheric section of the flight at
~23:25 UTC (marked as D) has a similar
characteristics as Events A, B and C and can easily be mistaken
for a plume. The only pronounced
difference is that the maximum of Hg concentration (and CO,
acetone and H2O mixing ratios) in
Event D coincides with dips in potential vorticity and O3 mixing
ratios, both at higher levelscharacteristic for the lower
stratosphere. Such dips in potential vorticity and O3indicate a
crossing of a
filament of tropospheric air in the stratosphere. The variation
of all mentioned species during such
crossing results from their strong gradients above the
tropopause [29]. Events of this type are thus not
related to surface emissions and have to be eliminated from
further consideration. Consequently, only
events embedded in air with a potential vorticity of less than
1.5 PVU (1 PVU = 106
m2Kkg1
s1)
and/or less than 150 ppb O3were considered.
Because we rely mostly on Hg and CO as plume tracers, only those
processes that emit Hg and CO,
such as biomass burning, will be detected. This includes also
collocated emissions of Hg and CO, CO2
or CH4, which applies for most of the urban and industrial
emissions. However, emissions from mining
and smelting, which emit hardly any CO, CO2 or CH4, will not be
detected (unless collocated with
other CO, CO2or CH4sources), because a suitable specific tracer
for these processes, such as SO2, is
not on the otherwise comprehensive list of CARIBIC in situ
measurements. Lacking in situ SO2
measurements also prevents the direct detection and
identification of volcanic Hg emissions. One such
SO2plume was detected during the descent to Frankfurt airport on
15 August 2008, using remote SO 2
sensing by a nadir looking differential optical absorption
system (DOAS) on-board CARIBIC and
remote sensing satellite [40]. Quantitative evaluation of this
plume in terms of Hg emission, however,
was not possible, because the DOAS measurement does not
providein situSO2concentrations and the
elevated Hg concentrations were documented by only one Hg
measurement point. Among the
substances measured in the whole air samples are tracers for
marine emissions, such as short-lived
bromine and iodine containing halocarbons, but the low sampling
frequency (28 samples taken over
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four intercontinental flights) makes them unsuitable as tracers
for the detection of the Hg plumes of
marine origin. 222Rn as a tracer for terrestrial Hg emissions
[22] is also not being measured on-board
CARIBIC. Consequently, in this study, we can only distinguish
between Hg plumes of biomass
burning or urban/industry origin.
The short encounter with the Kasatochi volcano plume (~5 min) in
2008 during the descent to
Frankfurt airport [40] also illustrates some practical limits of
our approach. The most severe limitation
is given by the low temporal resolution of the Hg measurements
of 515 min corresponding
to a ~75225 km flight distance. Statistically significant
Hgvs.CO, CO2and CH4correlations require
at least three measurements. Thus, only plumes larger than ~300
km can be captured by our approach,
which also means that many plumes encountered during the short
aircraft ascents and descents cannot
be resolved. The CARIBIC measurements start and stop at ~500 hPa
to prevent the contamination of
the CARIBIC system by polluted air in the boundary layer near
the airports [23]. Consequently,
information for the boundary layer, the most polluted part of
the troposphere, is missing in our dataset. In addition, the Hg,
CO, CO2and CH4data from ascents and descents through
quasi-horizontal
layers in the troposphere are likely to violate the assumption
of a plume being embedded in a
homogeneous air mass on which the Hgvs.CO, CO2and
CH4correlations are based.
Although almost all of the Hg plumes were accompanied by
elevated CO mixing ratios, seven of
them were observed during flight sections with nearly constant
CO mixing ratios. All of these plumes
were encountered over the equatorial Atlantic Ocean between 0
and 15 N during the flights to South
American destinations. They were embedded in background mercury
concentrations varying between
~0.95 and 1.3 ngm3, and the elevation above the background (Hg)
varied between ~0.25 and
0.45 ngm3
. In these events, elevated Hg concentrations were always
accompanied by elevatedhumidity, frequently also with clouds and
elevated NOy, as well as with low O3 mixing ratios;
frequently, only around 30 ppb. Such low O3mixing ratios are
typical for the marine boundary layer
over the equatorial Atlantic Ocean [41]. Backward trajectories
reveal contact with the equatorial
Atlantic Ocean surface. Satellite images of cloud cover indicate
that these events are due to the
convection of the air masses from the marine boundary layer at
the ITCZ. Elevated Hg concentration
in these events encountered in the upper free troposphere can
point to emissions of mercury by
ocean [42], but we lack highly resolved tracer data for air from
the marine boundary layer to
quantitatively describe them.
Despite these caveats and limitations, 98 encounters with plumes
with elevated CO mixing ratios
and simultaneously elevated Hg concentrations were counted
during 309 CARIBIC flights with valid
Hg measurements between May 2005 and June 2013. Taking into
account the number of flights to the
respective regions listed in Table 1, the probability of plume
encounters was highest during the flights
to South Africa with 85% of the flights. The second highest
probability of plume occurrence was over
East Asia with 46%, followed by flights to South Asia with 26%,
South America with 20% and North
America with 17%. The low frequency of plume encounters over
North America is partly due to the
high northern latitude of the flight routes of these flights,
which, at usual flight altitudes of
1012 km, results in a high proportion of stratospheric sections
with a potential vorticity >1.5 PVU.
However, the high frequency of plume encounters during the
flights to Osaka and Seoul with flight
routes at similarly high northern latitudes shows that this bias
alone cannot explain the low frequency
of plume encounters over Europe and North America.
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Figure 2. Overview of the data from Flight 158 from Frankfurt to
Guangzhou on
31 July 2006. (Top) Flight track and the locations of whole air
samples. Time series plots
are below: (uppermost panel): flight altitude (magenta) and
latitude (black), potential
vorticity (blue), sampling intervals (grey bars); (second panel
from top): mixing ratios of
CO (black), O3(green) and Hg concentrations (red); (middle
panel): mixing ratios of NO
(black), NOy (red) and total water content (blue); (second panel
from bottom): mixing
ratios of CH4 (blue), CH3Cl (olive green) and CFC12 (CCl2F2,
magenta)) in whole air
samples; (bottom panel): mixing ratios of acetone (green) and
CO2 (blue). The three
identified plumes are marked with A, B and C in the second panel
from top. Another event,
due to a crossing of a filament of tropospheric air within the
lower stratosphere, is marked
with D. Although similar to Events A, B and C, this event has no
relation to surface
emissions (see the text).
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For 56 plume encounters (out of 98), the Hgvs.CO correlations
were statistically significant at a
confidence level of at least 95%. For these correlations, data
from the same plume encountered twice
in the vicinity of an airport were combined, e.g., for a plume
near Guangzhou encountered during the
forward and return flight to Manila (Flights 203 and 204) or a
plume encountered near So Paulo
during Flights 123 (FrankfurtSo Paulo), 124 (So PauloSantiago de
Chile) and 125 (Santiago de
ChileSo Paulo). Seven extremely high Hg/CO slopes of 12.923.7
pgm3
ppb1 were connected
with physically unrealistic negative intercepts and are thus
eliminated from the data set, leaving 49
plume encounters with valid Hg/CO slopes. The geographic
distribution of these plumes is shown in
Figure 3.
Table 1. Overview of encounters with plumes with elevated Hg
concentrations and CO
mixing ratios.
Destination
Airport
Number of
Flights
Number ofPlume
Encounters
Number of PlumesWith Significant Hgvs.
CO Correlations
Median and Range ofHg/CO Emission Ratios
(pgm3
ppb1
)
South Africa 13 11 4 2.9 (2.27.5)
East Asia 101 46 31 8.2 (2.316.6)
South Asia 57 15 6 7.4 (5.010.0)
South America 90 18 5 1.3 (1.13.3)
North America 48 8 2 9.2 (6.9 and 11.4)
Figure 3. Geographic distribution and the extension of the
plumes with statistically
significant Hg vs. CO correlations. The magnitude of Hg/CO
emission ratios inpgm
3ppb
1is colour coded.
Most of the plumes with statistically significant Hg vs. CO
correlations (31 plumes) were
encountered over the East Asian region, and these are listed in
Table S1 (Supplementary Information).
Table S2 lists 18 plumes with statistically significant Hg vs.
CO correlations for all other regions.
Relative to the number of flights, the frequency of plumes with
statistically significant Hg/COcorrelations is the highest for the
South African and East Asian flights, with each being 31%,
followed
by flights to South Asia with 11%, South America with 6% and
North America with 4%. The high
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occurrence of plumes during the flights to South Africa in which
Hg does not correlate with CO is
caused by their large extension over a few thousands of km,
changing the Hg and CO background from
north to south hemispheric concentrations and the inhomogeneity
of the plumes. This will be discussed
later in Section 3.3.
The colour code of Figure 3 reveals a pronounced difference
between the Hg/CO emission ratios in
different regions. The Hg/CO emission ratios for plumes
encountered over East Asia range from 2.3 to
16.6 pgm3
ppb1(median 8.2 pgm
3ppb
1) and are similar to those over South Asia, ranging from
5.0 to 10.0 pgm3
ppb1 (median 7.4 pgm
3ppb
1). On the contrary, the Hg/CO emission ratios for
plumes observed during the flights to South America and
equatorial Africa range from 1.1 to
7.4 pgm3
ppb1, with a median value of 1.8 pgm
3ppb
1 (a median of 1.3 pgm3
ppb1 for South
America and 2.9 pgm3
ppb1 for equatorial Africa). These plumes include also two
plumes
encountered over the Atlantic Ocean, which, based on backward
trajectories, can be attributed to forest
fires in the USA, as will be discussed in Section 3.4. The range
and median of Hg/CO emission ratiosobserved over South America and
equatorial Africa is similar to the one for plumes observed at
Cape
Point in South Africa [21]. A compilation of previously
published Hg/CO emission ratios reported for
different processes and regions [29] shows that biomass burning
is characterized by ratios below
2 pgm3ppb1[43], whereas the ratios for urban/industrial
emissions tend to be around 6 pg m3ppb1
and higher. Applying these criteria to the Hg/CO emission ratios
shown in Figure 3 thus leads to the
conclusion that the plumes encountered during the flights to
South America and equatorial Africa
originate predominantly from biomass burning (see also [44]),
whereas the plumes observed over East
Asia, South Asia and North America are of industrial/urban or
mixed origin. This preliminary
classification is supported by the detailed discussion in
Sections 3.23.4.CO2 emissions are better known than those of CO,
and thus, the Hg/CO2 emission ratios have a
potential to provide a more accurate estimate of Hg emissions
[21]. Unfortunately, CO2 data were
available for only 69 (out of 98) of the encountered plumes, of
which nine were too narrow for
Hgvs.CO2 correlations. Statistically significant correlations of
Hgvs.CO2were only found for the
17 plumes listed in Table S3. These correlations will be
discussed in Section 3.5.
Mercury was also found to correlate frequently with methane at
Cape Point, which proved to be
useful for constraining the mercury emissions in South Africa
[21]. Methane data were available only
for flights since October 2010, and altogether, 26 correlations
of Hgvs.CH4could be calculated, of
which, only the six listed in Table S4 were statistically
significant. These will be discussed in
Section 3.6.
3.2. Plumes Encountered during the East Asian Flights
The East Asian plumes were encountered during the flights from
Frankfurt to Manila with a
stopover at Guangzhou and during the flights to Osaka and Seoul.
Apart from a few plumes over
Central Asia, most of them were encountered within a ~2000-km
distance from the airports at
Guangzhou, Osaka and Seoul. Data from one of the flights have
already been shown in Figure 2.
The Hg/CO emission ratios were 8.8 1.5, 11.3 2.0 and 7.49 1.0
pgm3
ppb
1
forEvents A, B and C, respectively. Air sample analyses (the
second panel from the bottom) show high
CH4in Sample 9 (Event A, sample numbers are marked in the
uppermost panel), 11 and 12 (Event B),
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and the highest level in Sample 14 (Event C). Acetonitrile as a
tracer for biomass burning is not
available for this flight section, but elevated CH3Cl mixing
ratios (dark green triangles in the second
panel from the bottom) indicate some influence of
biomass/biofuel burning in Samples 10, 11, 12 and
14, but not in Sample 9. Backward trajectories for the preceding
eight days for Samples 9, 11 and 12 in
Figure 4 all show that transport took place at a higher altitude
(
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Figure 4.Cont.
(b)
(c)
(d)
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Figure 5.The overview of the data from Flight 334 from Cape Town
to Frankfurt on 21/22
March 2011. The parameters displayed here are similar to those
in Figure 2. (Middle) The
time series plots additionally show the particle surface area
concentrations (green);
(second panel from the bottom) mixing ratios of SF6(magenta) and
N2O (green) in whole
air samples, as well as continuously measured mixing ratios of
CH4 (dark blue) and CO2
(light blue); (bottom) cloud water content (light blue) and
concentrations of particles
within the 412 nm size range (red) and larger than 12 nm
(black).
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A dense plume observed during Flight 300 from Osaka to Frankfurt
on 24 June 2010, covering a
distance of about 1000 km over the Korean Peninsula and Yellow
Sea, was characterized by CO
mixing ratios of ~240 ppb, Hg concentrations of 2.25 ngm3 and an
Hg/CO emission ratio of
4.4 pgm3
ppb1. High mixing ratios of biomass burning tracers, such as
CH3Cl, and of tracers of
anthropogenic origin, such as SF6, together with elevated levels
of pollutants, which may originate
from both biomass burning and anthropogenic processes, such as
ethyne and propane, point to the
mixed origin of this plume, both from anthropogenic processes
and from biomass burning. Backward
trajectories and a map of fire counts for 2024 June 2010 (not
shown, FIRMS (Fire Information for
Resource Management System) web fire mapper
http://firefly.geog.umd.edu, accessed on 28 October
2010), indicate that the biomass burning component originated
most likely from a region with a high
burning density in Shandong, Henan, Shanxi and Hebei provinces
of China and possibly from some
fires in southern Siberia. The anthropogenic component most
likely originated from the Chinese
provinces mentioned above.The plumes observed near Osaka during
two flights, 331 and 332, on 26/27 February 2011,
are characterised by similar CO mixing ratios as in June 2010,
but higher Hg concentrations
of ~2.7 ngm3, resulting in the higher Hg/CO emission ratios of
8.4 and 10.0 pgm
3ppb
1, which
point to urban/industrial origin. Air samples taken within the
plume had elevated mixing ratios of SF6
(~7.6 ppt), ethyne (~500 ppt), CH4 (~1870 ppb), CO2 (~397 ppm)
and several other hydrocarbons,
documenting the urban/industrial component of the plumes.
Biomass burning also contributed to
this plume, as evidenced by high CH3Cl mixing ratios of ~700
ppt. However, the high Hg/CO slope
of ~9 pgm3
ppb1 and the high SF6 mixing ratios imply urban/industrial
origin to be dominating.
Backward trajectories for the plume observed during Flight 331
show a fast transport from the westwith surface contact over
northern India, southern Pakistan and southern Iran (within ~3
days) and a
transport at high altitudes afterwards. Backward trajectories
for the plume observed during Flight 332
are similar, but because of their lower altitude contributions
from sources in China, cannot be ruled
out. However, based on the trajectories from Flight 331 and the
low probability of convection
in February, we deem northern India and southern Pakistan to be
the major source of the observed plumes.
A narrow plume observed during the descent to Seoul during
Flight 383 on 29 March 2012,
is characterised by very high CO mixing ratios of up to 357 ppb,
an Hg concentration of up to
2.49 ngm3and an Hg/CO emission ratio of 5.6 pgm
3ppb
1. Apart from high NOy and CO2, there
are no other measurements available to characterise this plume.
The medium Hg/CO slope indicates a
mixture of emissions from biomass burning and industrial/urban
emissions. The backward trajectories
are changing during the descent and point to northern China
or/and southern Siberia as possible
source areas.
Several plumes were also encountered over central Asia on the
way to Guangzhou and back,
especially during Flights 158, 161 (both flights were on 1
August 2006), 166 and 169 (both flights
were on 20 October 2006). The events during the outward bound
flight on October 20 at
3:534:53 UTC and during the return flight on the same day at
17:4920:29 UTC have very similar
Hg/CO emission ratios of 3.5 0.7 and 3.2 1.1 pgm3
ppb1, respectively, and originate, with little
doubt, from one and the same plume. This plume is analysed in
detail by Bakeret al.[46]. The low
Hg/CO emission ratio and elevated acetonitrile, CH3Cl and CH3Br
mixing ratios suggest that the
plume originates partly from biomass/biofuel burning, whereas
elevated mixing ratios of C2Cl4 and
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toluene, which are used as solvents, indicate anthropogenic
contributions. Backward trajectories for
the samples taken in this plume pass over Afghanistan, Pakistan
and a region of Northern India, where
extensive fire activity was recorded during 1729 October 2006
[46]. Another plume with an Hg/CO
emission ratio of 11.2 3.4 pgm3
ppb1and very high acetone mixing ratios was encountered
during
Flight 161 on 1 August 2006, at 22:0723:07 UTC. No air samples
were taken during this event.
Backward trajectories pass at high altitude (
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Figure 6.Cont.
(b)
(c)
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Figure 6.Cont.
(d)
Figures 5 and S1 show a typical example of the data obtained
during Flight 334 from Cape Town to
Frankfurt on 20 and 21 March 2011. The second panel from the top
of the data time series in Figure 5
shows somewhat elevated CO mixing ratios of ~100 ppb after
ascent, decreasing to a southern
hemispheric background of ~ 75 ppb after 18:30 UTC. CO then
increases in the course of the flight up
to a maximum of ~240 ppb between 22:45 and 23:10 UTC to
subsequently decrease, with another
smaller peak, with a maximum of ~175 ppb around 0:20 UTC, to
~125 ppb, before the aircraft crosses
the tropopause into the lower stratosphere at ~1:00 UTC. The NOy
mixing ratio and particle surface
area concentration (the third panel from the top) display a
similar pattern, even in the finer structure,
whereas CO2and CH4show only a broad maximum with a somewhat
different shape, peaking shortly
before 23:00 UTC. The SF6mixing ratio increases gradually from
~7.2 ppt after ascent to ~7.3 ppt
before entering the stratosphere. Although small, this increase
documents that all plume observations
are embedded in a broad gradient between southern and northern
hemispheric air masses [47]. Mixing
ratios of ethane, propane and ethyne, shown in Figure S1,
broadly follow the CO pattern, but high
mixing ratios of short-lived n-butane and i-butane around 21:15
UTC and at 23:00 UTC indicate the
admixture of freshly polluted air. The CH3Cl mixing ratio
(Figure S1, lowermost panel) of ~ 650 ppt in
the tropospheric section of the flight is substantially higher
than the background mixing ratio of
~550 ppt, and this implies a large-scale influence of biomass
burning. The highest CH3Cl mixing ratios
of almost 700 ppt are found in two samples taken after midnight.
They coincide with elevated mixing
ratios of CO, ethane, propane, ethyne and NOy (Figure 5, middle
panel), but the short-lived butanes
have almost disappeared. Such a coincidence is characteristic
for aged air from another regional
biomass burning plume. Consequently, the CO bulge over
equatorial Africa has to be viewed as a
composite of several regional plumes. Backward trajectories,
shown in Figure 6 for 2122 UTC (a),
2223 UTC (b), 2324 UTC (c) and 01 UTC (d), support this view by
pointing to different source
regions in different sections of the flight. Several of the
trajectories exhibit rather fast upward transport
from the boundary layer and lower troposphere (purple and red
colours). Cloud water content
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(Figure 5, bottom panel) in several sections of the flight is a
sign of convective activity in these areas.
The fire map displayed in Figure 7 and the trajectories show
that emissions from biomass burning
have, to a varying degree, influenced all observations between
~21:00 UTC on 20 March to
~1:00 UTC on 21 March.
The correlation of Hg vs. CO was statistically non-significant
for the whole plume starting at
21:16:30 and ending at 01:01:30 UTC, as well as for sections of
it, such as between 23:56:30 and
00:56:30 UTC or between 21:21:30 and 23:56:30 UTC. The
correlation of Hgvs.CO2was statistically
significant for the whole plume (239.4 94.4 pgm3
ppm1 at > 95% level), but statistically
non-significant for the sections mentioned above.
Figure 7. The map of fire counts for the period from 12 to 21
March 2011
(http://rapidfire.sci.gsfc.nasa.gov/firemaps/, accessed on 10
October 2013).
In summary, the scarcity of statistically significant Hgvs.CO
and Hgvs.CO2correlations in the
plumes observed over equatorial Africa is a result of several
factors. The plumes are embedded in
broad north-south gradients, which violates the assumption of a
constant background. Due to their
large extent, they consist of a multitude of overlapping smaller
plumes from different regions and
sources and are thus not homogeneous. In addition, the CO
enhancements (CO) against the
background are rather small, varying between ~45 ppb during
Flights 290 and 291 to Cape Town on
27 and 28 October 2009, to ~165 ppb during Flights 333 and 334
to Cape Town on 20 and 21 March
2011. Assuming that the plumes originate predominantly from
biomass burning with a typical Hg/CO
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emission ratio of 1 pgm3
ppb1, the CO enhancements of this magnitude would produce Hg
enhancements of only 0.045 to 0.165 ngm3. Such enhancements are
difficult to detect with a
precision of 0.05 ngm3of the mercury measurements, even if the
background were constant and the
plumes homogeneous. For all of these reasons, the Hg/CO emission
ratios derived from these flights
will be substantially more uncertain than their statistical
uncertainty stated in Table S2.
Figure 8. An overview of the data from Flight 348 from Bogot to
Frankfurt on
17 June 2011. The same parameters are displayed as in Figure 5.
Additionally, total water
content (dark blue) is shown in the bottom panel.
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Figure 9. (a) eight-day backward trajectories for whole air
Sample 6 from the CO peak
encountered around 17:20 UTC during Flight 347 from Frankfurt to
Bogota on 16 June 2011;
and (b) for whole air Sample 20 from the CO peak encountered
around 8:05 UTC during
Flight 348 from Bogota to Frankfurt on 17 June 2011. (c) The map
of the fire counts for 1019
June 2011 (http://rapidfire.sci.gsfc.nasa.gov/firemaps/,
accessed on 10 October 2013).
(a)
(b)
(c)
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Three of the four statistically significant Hg vs. CO
correlations for plumes observed over
equatorial Africa yield Hg/CO emission ratios of 2.183.36
pgm3
ppb1, which, again, points to the
predominant contribution of emissions from biomass burning.
3.4. Plumes Observed during the Flights to and over South
America
The plumes encountered over South America during the flights to
So Paulo and Santiago de Chile
were analysed in detail by Ebinghaus et al. [48]. Here, we would
only note that based on their
chemical signature, backward trajectories and fire maps, these
plumes could be unequivocally
attributed to biomass burning in the Amazon Basin and its
outskirts.
The encounters at 16:47:30 to 17:47:30 during Flight 347 from
Frankfurt to Bogot on 16 June
2011, and at 7:35:308:45:30 during the return Flight 348 on 17
June 2011, both above the middle of
the Atlantic Ocean at latitudes ranging from 31 to 43N, are
probably due to the same plume.
An overview of the data from Flight 348 (Figure 8) shows a sharp
CO peak with ~325 ppb at
~8:00 UTC, accompanied by peaks in NOy, aerosol surface area,
CH4and a small peak of SF6, the last
one originating from anthropogenic emissions. The CO peak
coincides also with the highest CH3Cl,
ethane, propane, n-butane, i-butane and ethyne mixing ratios
(Figure S2). The low Hg/CO emission
ratios of 1.5 0.6 pgm3
ppb1for this flight and 1.3 0.4 pgm
3ppb
1for Flight 347 and the peak
CH3Cl mixing ratio indicate that pollutants from biomass burning
are by far the most predominant
component of these plumes. Backward trajectories in the upper
panel of Figure 9 for Samples 6 and
20, taken within the CO peaks observed during Fights 347 and
348, respectively, show a high level
transport from U.S. and north-western Mexico. Satellite cloud
images point to convective activity over
the south-eastern U.S., the Great Plains and north-western
Mexico. The map of fire counts for
1019 June 2011, in the lower panel of Figure 9, shows that
numerous fires in the southeast U.S. might
be the major source of the observed plumes, with a possible
contribution of fires in Southern
California and north-western Mexico.
3.5. Hg/CO2Emission Ratios
Hg/CO2emission ratios are potentially more useful for
constraining the mercury emissions, because
CO2 inventories tend to be more accurate than those of CO [21].
Unfortunately, only a few Hg/CO2
emission ratios have been reported, so far. Table S3 displays
the events with significant Hg vs.CO2correlations. CO2 data were
available only for 46 plume encounters. Among these, for only
17 encounters, the Hgvs.CO2correlations were statistically
significant (significance level 95%). The
yield of statistically significant Hgvs.CO2correlations is thus
somewhat smaller than for Hgvs.CO, and
the significance of these correlations with mostly only 95%
tends also to somewhat smaller values. Eleven
of the statistically significant correlations were found for
plumes with significant Hgvs.CO correlations.
The Hg/CO2 emission ratios vary over a broad range, from 14.4 to
964 pg m3
ppm1, and those
observed during the flights to East Asian destinations vary
between 107 and 964 pg m3
ppm1.
The Hg/CO2emission ratio from the plume observed during Flight
334 to Frankfurt immediately after
ascent from Cape Town on 21 March 2011, and during Flight 373 to
Chennai after ascent from
Frankfurt on 16 January 2012, also fit the range of East Asian
plumes. The lowest Hg/CO 2emission
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ratios of 14.4 and 21.9 pgm3
ppm1were both derived from plumes encountered during Flights
329
and 334 over equatorial Africa on 24 February 2011 and 21 March
2011, respectively.
The low Hg/CO2emission ratios in the plumes of equatorial Africa
are comparable to the median of
34.1 pgm3
ppm1 (average: 62.7 80.2 pgm
3ppm
1) of emission ratios observed in the plumes
encountered at Cape Point, which, according to their Hg/CO
emission ratios, seem to originate
predominantly from biomass burning [21]. The Hg/CO2 emission
ratio from the only plume clearly
attributed to biomass burning near Cape Point was somewhat
higher with 109 27 pgm3
ppm1[49],
but this is comparable to 131 53 pgm3
ppm1, observed in the plume from biomass burning
in the south-eastern U.S. in June 2011 (Flights 347 and 348).
Based on the coal mercury content of
0.150.45 gHgg1 and a flue cleaning efficiency for mercury of
50%90%, Brunke et al. [21]
predicted an Hg/CO2emission ratio to be within the range of 230
pg m3
ppm1. The Hg content in
coal consumed in China varies from 0.027 to 0.369 gg1[50] and is
not much different from that in
South Africa. The flue cleaning efficiency for mercury in China
is with up to 57% somewhat lower [50],but this difference cannot
explain the Hg/CO2emission ratios larger than 100 pgm
3ppm
1observed
over East Asia, Europe and at Cape Point in South Africa [21].
If confirmed by further measurements,
high Hg/CO2emission ratios would imply a substantial
contribution of emissions from other sources
than coal burning.
3.6. Hg/CH4Emission Ratios
Mercury also frequently correlated with CH4 in the plumes
observed at Cape Point, and the
resulting Hg/CH4 emission ratios helped to constrain the mercury
emissions in South Africa [21].
Continuously measured CH4data were available only for 26 plume
encounters, and of these, only six
provided a statistically significant Hgvs.CH4correlation at a
confidence level of at least 95%. The
emission ratios listed in Table S4 vary between 4.8 and 41.4
pgm3
ppb1. The only Hg/CH4emission
ratios available for comparison are derived from observations at
Cape Point and are centred in the
range of up to 6 pgm3
ppb1 [21]. The plume encountered during the flight to Cape Town
with an
emission ratio of 4.8 pgm3
ppb1falls into this range. All other plumes in Table S4 were of
mixed or
industrial/urban origin, and they have higher Hg/CH4emission
ratios.
4. Conclusions
Over 100 plumes with elevated mercury concentrations were
encountered during the tropospheric
sections of the CARIBIC flights from May 2005, until June 2013.
In 98 of them, elevated Hg was
accompanied by elevated CO mixing ratios. Several Hg plumes
without a simultaneous increase in CO
were all encountered over the equatorial Atlantic Ocean during
the flights to South America and were
attributed to the convection of the marine boundary air at the
ITCZ. Hg correlated as statistically
significant with CO in more than 50% of the observed plumes and
with CO 2 in about 30% of the
plumes for which CO2data were available. Ample ancillary data on
the chemical fingerprint of the air
within these plumes and backward trajectories provide additional
means to identify the origin and the
type of the source.
Extensive mercury plumes over equatorial Africa were observed
during all flights between
Frankfurt and South Africa. These plumes, which extend over
thousands of kilometres, are embedded
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in north-south gradients of mercury, CO and CO2 and consist of a
number of overlapping smaller
plumes. Due to the changing background, the inhomogeneity of the
plumes and the low precision of
the Hg measurements, only a few of the plume encounters provided
statistically significant Hgvs.CO
correlations. Most plumes were observed over East Asia, and
relative to the number of flights to East
Asian destinations, the yield of plumes with statistically
significant Hg vs.CO correlations was on par
with the African flights. Lower yields of plume occurrence were
found for flights to South America
and to South Asia. Only two plumes were encountered over North
America and one over Europe.
The Hg/CO emission ratios derived from these correlations are
consistent with the previously
published data compiled by Slemret al.[29] and have a smaller
values of ~1 pgm3
ppb1for plumes,
which we clearly could attribute to biomass burning using
backward trajectories, fire count maps and
the presence of chemical tracers for biomass burning, such as
CH3Cl and acetonitrile. Larger values of
~6 pgm3
ppb1 and more were found for most of the other plumes. Backward
trajectories and the
presence of man-made tracers, such as C2Cl4 and SF6 in several
of these plumes suggest emissionsfrom urban/industrial sources.
Both types of chemical tracers were present in several plumes,
with
Hg/CO emission ratios between 1 and 6 pgm3
ppb1, showing their mixed origin.
Many of the plumes were transported over large distances from
the area of their origin to the place
of their observation. Backward trajectories point to major
source areas in equatorial Africa, East Asia,
South America and South Asia. The emissions from equatorial
Africa and South America are clearly
dominated by biomass burning. The East Asian emissions originate
from a large area of East Siberia,
Korea, China, the Philippines and the Indochinese Peninsula.
They are mostly of urban/industrial
origin with a varying contribution from biomass and biofuel
burning. The South Asian emissions
originate mostly from the Indo-Ganges region of northern India.
Like the East Asian ones, they are amixture of emissions from
industrial/urban sources and biomass/biofuel burning. Other mercury
source
areas were also identified: the Middle East, a region along the
northern perimeter of the Black Sea, the
Mediterranean Sea and the south-western U.S. The Hg/CO emission
ratio and the plume fingerprint for
the Middle East region suggests industrial/urban emissions to be
dominating. The emissions whose
origin area was located toward the northern coast of the Black
Sea and toward the Mediterranean had
both a substantial contribution from biomass burning. Biomass
burning was the major component of
the emissions from the south-eastern U.S. during one event. We
caution that since only a few plumes
were attributed to each, the Middle East, Europe and the U.S.,
no firm conclusions can be drawn for
these areas. We note also that the forest fires at northern
mid-latitudes occur in summer when
convection processes, which carry them to cruising altitudes,
are the most active. Consequently, our
observations for these areas are biased in favour of biomass
burning and neglect emissions in other
seasons, such as, e.g., from residential heating in winter.
Only a few Hg/CO2 and Hg/CH4 emission ratios have been reported,
so far. The range of the
Hg/CO2 emission ratios from the CARIBIC flights is comparable to
the range observed at
Cape Point [21]. The Hg/CO2emission ratios of 107964 pgm3
ppm1observed in the plumes over
East Asia, however, are substantially higher than 230 pgm3
ppm1, calculated by Brunkeet al.[21]
for coal burning. If confirmed by further measurements, the
higher observed than calculated Hg/CO 2
emission ratios would imply other substantial Hg sources in
addition to coal burning.
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Acknowledgments
We would like to thank Lufthansa and all members of the CARIBIC
team for their continued effort
to keep running such a complex project. We thank especially
Dieter Scharffe, Claus Koeppel and
Stefan Weber for the day-to-day maintenance and operation of the
CARIBIC container. Funding from
the European Community within the GMOS (Global Mercury
Observation System) project and from
Fraport AG is thankfully acknowledged. We acknowledge the use of
FIRMS data and imagery from
the Land Atmosphere Near-real time Capability for EOS (LANCE)
system operated by the
NASA/GSFC/Earth Science Data and Information System (ESDIS) with
funding provided by
NASA/HQ.
Author Contributions
All authors are members of the CARIBIC team and contributed to
the production of the data onwhich the paper is based. Franz Slemr
calculated the emission ratios, and Peter van Velthoven made
the meteorological analyses for each flight and calculated the
backward trajectories. All authors
discussed the results of the manuscript in all stages of its
preparation.
Conflicts of Interest
The authors declare no conflict of interest.
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